The specific aim of this study is to assess the effect of heparin therapy during pregnancy on bone density. The study is prospective and observational. Potential subjects include women receiving heparin therapy prophylactically for a history of thromboembolism and patients receving therapy for deep vein thrombosis, thrombophlebitis or any condition requiring heparin. Blood and urine samples and a dietary intake history are taken at 20 weeks gestation, 6 weeks postpartum, and 6 and 12 months postpartum. Bone density is measured at 20 wks gestation, following delivery and 6 and 12 months postpartum. The spine is measured after delivery.